This invention relates to improvements in a power steering system, and more particularly to the power steering system in which a steering assist force is developed by a hydraulic power cylinder.
A power steering system of the kind wherein a steering assist force is developed by a hydraulic power cylinder is disclosed in Japanese Patent Provisional Publication No. 2001-1918. This power steering system includes hydraulic lines (44a, 44b) which are connected respectively with left-side and right-side hydraulic pressure chambers of the hydraulic power cylinder. Relief valves (54a, 54b) are disposed respectively in hydraulic lines. These relief valves are adapted to allow hydraulic fluid to bypass the hydraulic line so as to direct the hydraulic fluid to another oil passage when the internal pressure of a hydraulic circuit becomes a certain level or higher, thereby preventing the hydraulic circuit from becoming into an overload condition.
However, in the above conventional power steering system, the internal pressure of the hydraulic circuit for connecting the left-side and right-side hydraulic pressure chambers of the hydraulic power cylinder becomes a level of generally atmospheric pressure during non-steering assisting in which an oil pump is not operated, and therefore the hydraulic power cylinder is unavoidably readily moved when kickback is input to the hydraulic power cylinder from road surface so that steering tends to become unstable. Additionally, since the pressure within the hydraulic circuit becomes the level of generally atmospheric pressure during the non-steering assisting, a time is required until the hydraulic pressure within the hydraulic circuit sufficiently rises after start of operation of the oil pump during steering assisting, so that operational delay of the hydraulic power cylinder occurs. More specifically, although hydraulic fluid within the hydraulic circuit is an incompressive fluid, air bubbles mixed in hydraulic fluid cannot sufficiently disappear until the hydraulic pressure becomes a set level or higher. Accordingly, the pressure of the hydraulic power cylinder cannot abruptly rise until the hydraulic pressure within the hydraulic circuit sufficiently rises so that air bubbles disappear. This causes the operational delay of the hydraulic power cylinder.
Further, it is necessary to set the valve opening pressure of the above-mentioned relief valve at a high level in order to prevent the relief valve from opening under the internal pressure of the hydraulic circuit during normal steering, and therefore the internal pressure of the hydraulic circuit rises to the level of the valve opening pressure of the relief valve at the highest. In case of making steering assist under this condition, in order to generate a steering assist force upon developing a pressure differential between the left-side and right-side hydraulic pressure chambers of the hydraulic power cylinder, the oil pump is required to provide a higher pumping pressure than that under a condition where the internal pressure of the hydraulic circuit is low. As a result, it is necessary to set a driving current for an electric motor for the oil pump (reversible pump) at a higher level. This causes an increased electric power consumption. Additionally, setting the driving current at the higher level increases a heating value of the electric motor and driving elements, thereby providing a fear of putting the hydraulic circuit into an overheat condition.